This invention relates to operational amplifier circuits employing complementary metal-oxide-semiconductor transistors and more particularly, to such circuits having improved frequency compensation.
In order to assure operational stability for an operational amplifier having multi-gain stages, the phase shift at its output, under open loop conditions, should not exceed 180.degree.. To achieve this, the operational amplifier must be internally compensated.
One prior method for providing such compensation, was to use a compensating capacitor (C.sub.c) in the lead between the output and input of the operational amplifier. However, this arrangement resulted in the creation of a transfer function zero at (gm/C.sub.c) (where gm is the transconductance factor for the circuit) in the frequency domain by virtue of direct feed forward. For MOS circuits, this zero is located within the bandwidth of interest and decreases the output phase shift. At the same time, it prevents the output magnitude from rolling off and can cause instability in the amplifier. One prior scheme for solving this problem involved the use of a noninverting buffer amplifier in the feedback path, which served to avoid feed forward effects that originally created the zero. This approach is described in the IEEE Journal of Solid State Circuits, Vol. SC-11, pp. 748-753. However, a disadvantage with this scheme was that the buffer amplifier consumed significant and excessive amounts of power. Another approach to solving this problem, was to reduce the value of the compensating capacitor (C.sub.c). The effect of this approach is to shift the frequency of the transfer function zero to a higher frequency outside the region of interest for the particular operational amplifier. This scheme would appear to be reasonable for operational amplifiers having high open loop gain, such as CMOS operational amplifiers. However, the drawback of this technique is that high frequency power supply noise rejection is poor. This is due to the fact that at low frequencies, any noise on the ground power conductor is not amplified by the input element of the output stage. However, with increasing frequency, the capacitor C.sub.c creates a smaller impedance between internal nodes which creates an increased gate to source voltage on the input elements and results in decreased power supply noise rejection.
It is therefore one object of the present invention to solve the problem of providing for effective internal frequency compensation in CMOS operational amplifiers.
Another object of the invention is to provide an improved CMOS operational amplifier circuit with adequate frequency compensation and reduced power dissipation.
Another object of the invention is to provide a CMOS operational amplifier circuit that improves the power supply noise rejection ratio of the amplifier.
Still another object of the present invention is to provide a CMOS operational amplifier particularly well adapted for use in large scale integrated circuit devices.
Another object of the invention is to provide an increased gain bandwidth product for the operational amplifier.